This invention relates to a method for leak testing sealed containers and/or packages, particularly sealed packages containing at least one liquid component therein.
Many manufactured items must be tested before shipment to verify that they will adequately perform as required when they are made available to a customer or end-user. For example, when a packaged consumer good, such as pre-packaged contact lens, is required to contain a given amount of liquid in a leakproof fashion, such a package is typically tested to ensure a leakproof seal. Many prior art leak testing techniques exist in which sealed containers are placed in a chamber and subjected to a vacuum. Once the desired vacuum is obtained, the chamber is sealed from the atmosphere and any recorded change of pressure, within the vacuum chamber, is used to determine if the package is leaking in excess of a desired rate. The presumption is that the change in pressure is due to the container leaking its liquid contents into the chamber where the liquid is vaporized. See for example U.S. Pat. No. 5,907,093 issued to Martin Lehmann on May 25, 1999 titled, xe2x80x9cMethod For Leak Testing And Leak Testing Apparatusxe2x80x9d.
An alternate method is to determine how much mass has to be pumped from the chamber to maintain the partial vacuum level. The latter method is called xe2x80x9cflowxe2x80x9d rather than xe2x80x9cvacuum decay.xe2x80x9d
Any rise in pressure, within the vacuum chamber, is indicative of vaporization of available liquids whether from a leak in the sealed container, a chamber leak, or water vapor emitted from the walls of the test chamber or other surfaces, a phenomena known as outgassing. Known state-of-the-art xe2x80x9cpressure risexe2x80x9d moisture detection instruments work by this principle.
If the pressure increase, in a vacuum decay test, is relatively small, then there must have been very little liquid present in the chamber, and the part is considered satisfactory for shipment. If the pressure increase is large, it is then assumed that there must have been a significant amount of the liquid (e.g. a significant source of gas), and the product being tested is rejected for shipment. Of course, to make an accurate determination it is necessary to ensure that the pressure inside the chamber falls below the vapor pressure of the liquid in the sealed package being tested and that the pressure increase came from the liquid in the package rather than from chamber surfaces.
None of the known prior art for detecting leaks in sealed containers, having at least some liquid therein, has been effective in distinguishing between pressure increases caused by chamber leaks, or outgassing, and pressure increases resulting from vaporization of liquid drawn from a leak in the sealed container. The traditional method of simply looking for a pressure change to determine the quality of a part may lead to false rejects of good products. In addition, if the acceptable leak rate is very small, the system may be unable to distinguish pressure changes due to package leakage, from the pressure changes resulting from the other causes described above.
Accordingly, it is an object of the present invention to provide a package leakage test method to test the integrity of sealed containers having at least one liquid component therein that is capable of differentiating between pressure increases caused by system outgassing and pressure increases caused by a leaking container.
The present invention teaches a method of detecting moisture leakage from sealed packages containing liquid, such as contact lens packages. The package to be tested is placed within a vacuum chamber and the chamber pressure is decreased to a pressure that is below the vapor pressure of the liquid contained within the package.
After a predetermined amount of time, the vacuum chamber is sealed and a leak is detected by measuring the pressure increase, as a function of time, within the chamber. The vacuum, within the chamber, will cause liquid to leak from a defective package and the vaporization of the leaking liquid will cause a pressure rise in the vacuum chamber.
However, a pressure increase can not only occur from liquid being sucked out of a leaking package and boiling off into the chamber, but also leaks in the chamber walls, door, or moisture evaporating off of the chamber walls may cause a measurable increase of pressure within the vacuum chamber.
By the present invention, chamber pressure increases from sources other than package leaks are distinguished from package leak pressure increases by analyzing the vacuum chamber pressure increase, as a function of time, as represented by a recorded pressure vs. time curve. Pressure increases caused by the system are represented by a substantially smooth curve pressure vs. time curve. However, chamber pressure increases caused by package leaks create a xe2x80x9crippledxe2x80x9d or substantially xe2x80x9cbumpyxe2x80x9d curve. This is because bursting bubbles of the boiling liquid, escaping from the package, cause rapid and substantially instantaneous pressure increases, or bumps in the curve. A method such as the least squares method, and other linear and non-linear methods, which are well known in the art, may be employed to quantify the amount of curve ripple. By using curve ripples to detect leaks, rather than overall chamber pressure increase, the method of the present invention is capable of more accurately detecting leaks.
Accordingly, it is an object of the present invention to provide a leak test method to test the integrity of sealed packages, having at least one liquid component therein, that is capable of differentiating between chamber pressure increases caused by system outgassing, or other events not related to a leaking package, and pressure increases caused by package leakage.